1. Field of the Invention
This invention relates to an access control device and a method for a dynamic memory device requiring periodic refreshing of the memory content, and more particularly, a method and a device capable of accessing at a definite period irrespective of the refreshing operation.
2. Description of the Prior Art
As is well known in the art, in case of, for example, 256 K-bit DRAM, a dynamic memory device is required to refresh at a period of about 4 milliseconds for the purpose of holding the memory content. During an interval in which the refreshing is made it is impossible to access from outside. Accordingly, when an external access request is made during the refreshing interval, it is necessary to cause the access request to wait until the refreshing is completed.
Consequently, in a prior art circuit for controlling access of a dynamic memory device where a refresh request and an external access request occur simultaneously, a priority is given to the refresh request, whereas when these requests do not occur simultaneously, the refresh request and the access request are accepted according to their received order so as to permit an access corresponding to a given access request.
However, when a refresh request and an external access request occur simultaneously by merely giving the priority to the refresh request, where there is a device periodically producing access requests, for example, where a typewriter is connected, the printing device must be designed by taking into consideration the refresh period which delays the printing speed.